Sunday, March 24, 2013

Location, Location, Location

The incoming storm for late tonight and tomorrow will bring a proverbial mess and mixed bag of precipitation to the region -- rain and snow are likely through the region starting after Midnight tonight and continuing through a good chunk of Monday before tapering off late in the afternoon and on Monday evening.

This storm, a mixed blessing and mixed bag, will present a ton of forecast challenges. First, temperatures at the surface and in the bottom layers of the atmosphere may not be cold enough for snow to stick unless precipitation rates are intense enough. The mesoscale models like the NAM, RGEM, SREF, which have been awful at this over the past few weeks, continue to suggest the possibility for heavier snow as they are more robust on precipitation and low pressure intensity than the GFS and Euro. This has been the general theme over the course of the past six weeks with those models, particularly the NAM.

None of the models have been perfectly consistent with track and detail but they all generally show a transfer of low pressure energy from a system moving through the Midwest to a coastal that's expected to develop late tonight and early tomorrow off of the Carolina or Virginia coastline. From there, the track and intensity vary, with the mesoscale models closer to the coast while the Euro/GFS are farther removed and have a slightly weaker storm track.

Precipitation is expected to move in after Midnight in the city, likely as a mix of wet snow and rain from Philadelphia on east, with snow falling west of the city. South and west of the city will see precipitation first, likely after 8 or 9 PM but it will take a while to get into Philadelphia.

The bulk of the storm system moves through during the morning and midday hours, with steady precipitation that could be heavy for a time across portions of New Jersey. The meso models, like the NAM below, show that precipitation shield farther north than the GFS...this is important to note because if we do see heavier precipitation push north and west the potential heavier snowfall totals does exist to the north and west of the city. Only the meso models are showing it. Again, remember their track record the past six weeks. It's not a good one.

For those of you along and east of 95, this is going to be a mix of rain and snow for the most part. It may snow on the front and tail end, with a mix in the middle. The closer to the Shore you are the higher the likelihood of all rain falling. North and west of the city could see a mix of rain and snow in lower elevations or snow that struggles to stick, especially if precipitation is lighter in intensity and falls along the Euro's train of thought. In the March 7th storm, the Euro was closest on precipitation although it missed on precipitation type (predicting snow/mix/snow instead of rain that ended as a few flakes).

Precipitation winds down during the evening hours from west to east, with scattered showers through the night, some of which may be mixed with snow.

At minimum, Monday will be a miserable day from a temperature and precipitation standpoint. Highs will probably not get to 40, with 30's a likely bet for the region. We will pick up between a half and one inch of liquid precipitation in the city, with about an inch at the Shore, and less precipitation north and west.

What determines just how much snow falls is dependent on precipitation intensity -- a heavier shield of precipitation will help generate a bit more pull of cold air from aloft towards the surface, allowing for more snow to fall to the surface. Hillsides will also do better than valleys with this storm -- a couple of degrees matter when it comes to snowfall so elevation may be a bit of a friend with this storm.

Putting all of that together, we're looking at a general coating to an inch for everyone along and east and south of I-95. Starts as a mix, transitions over to rain during the mid morning, ends as rain or a mix. If it starts as snow across I-95, totals could exceed an inch. North and west will see a general one to three inch accumulation -- some hills may see four inches in Chester County or in Bucks County -- those favored areas that typically get a bit more snow than others. Mount Airy and Chestnut Hill will fall into that one to three range due to their elevation as the high points in the city. This is not a high confidence call but it's a safe call given the usual factors working against late season snowfall -- sun angle, marginal temperatures at the surface -- caution is best served. The good news is that precipitation does move in overnight, which will help lay some snow down north and west of the city. Not all of it will stick after sunup once the sun starts to work through the clouds and melt some of that snow off during the day.

We will keep an eye on this and we will revise forecast IF it's needed...that said, erring on the side of caution puts us at this point for a forecast for snowfall accumulation.